1. Filed of the Invention
The invention relates to a capacitive level measurement and detection device for measurement and detection of the level of a medium in a container or the like, with a sensor element, with a reference element, and with an electrical circuit which contains at least one AC voltage source and an evaluation unit and which is connected, on the one hand, to the sensor element, and on the other, to the reference element, the sensor element and the reference element being made and arranged such that the medium with the level to be measured or detected can influence the impedance between the sensor element and the reference element, and thus, the electrical alternating current between the sensor element and the reference element (and in the electrical circuit).
2. Description of Related Art
Initially, the designation level measurement device or level detection device was chosen because it can, depending on the level of a medium in a container or the like being measured, generate an analog measurement signal, and also can detect only a certain, given level of a medium in a container or the like, therefore, generating a digital measurement signal when the given level has not reached/has been reached.
Generally, a capacitive level measurement and detection device for measurement or detection of the level of a medium in a container is used in a narrower sense, for example, in a tank. Initially, it is noted that the expression “for measurement and detection of the level of a medium in a container or the like,” is intended to express that it can be a matter not only of the level of a medium in a container in a narrow sense, but that it can also be a matter of measuring and detecting the level of a medium, for example, in a trough.
For the capacitive level measurement and detection device under consideration, first of all, it is not important how the sensor element, on the one hand, and the reference element, on the other, are made; what is important is simply that the sensor element and the reference element are made and arranged such that the medium with a level to be measured or detected can influence the impedance between the sensor element and the reference element. The sensor element and reference element must, therefore, be made and arranged such that there is an intermediate space between the sensor element and reference element, and the medium with a level which is to be measured and detected can more or less travel into this intermediate space. Generally, at least the sensor element is one which has a lengthwise extension in the vertical direction so that the medium with a level which is to be measured or detected more or less touches or surrounds the sensor element at least partially depending on the instantaneous level.
The sensor element and the reference element can be components separate from the container. It is also possible to use a container wall as the reference element. It is also possible to implement the sensor element and also the reference element by the container wall or by parts of the container wall; of course, then, the sensor element and also the reference element must be galvanically separated from one another.
Capacitive level measurement and detection devices of the type under consideration and the measurement and evaluation processes used in this connection are common in the prior art. In this respect, reference is made to German patent disclosure documents 38 12 687; 195 02 195; 198 13 013 (corresponding to U.S. Pat. No. 6,225,710); 100 08 093 (corresponding to U.S. Pat. No. 6,823,730); 101 53 298; 101 55 131; 102 05 445 (corresponding to U.S. Pat. No. 6,782,747); and 102 50 523. In particular, German patent disclosure documents 38 12 687; 195 02 195; 100 08 093 (corresponding to U.S. Pat. No. 6,823,730); and 101 55 131 also disclose sensor elements as can be used in the capacitive level measurement and detection devices under consideration.
One problem is common to the known capacitive level measurement and detection devices of the type under consideration; it is explained below.
When using the known capacitive level measurement and detection devices, adherence of the medium, the level of which in the container or the like is to be measured or detected, to the sensor element and/or the reference element cannot be prevented. Here, adherence means residues of the medium which have remained on the sensor element and/or the reference element, if after the level has once been reached, the medium has returned. In any case, when these adherences are low-resistance, it can no longer be adequately distinguished between these adherences and the voluminous medium with a level which is to be to be measured or detected. This is due to the unfavorable ratio of the relatively high impedance of the sensor element and/or of the reference element and the relatively low impedance of the adherences and of the voluminous medium. The relatively low impedance of the adherences and of the voluminous medium results from the fact that at the frequencies of the AC voltage provided by the AC voltage source used, for example, a frequency of 2 MHz, the impedance is ohmic-capacitive. The ohmic portion of these impedances also results largely from the ion conductivity of the adherences and of the voluminous medium.